Log periodic antenna with foreshortened dipoles

ABSTRACT

A log periodic antenna comprising foreshortened dipoles, each having a profile corresponding to the interior cross section of a hollow ridged waveguide. One or more of the elements of the array, usually those which are resonant at relatively low frequencies (HF and low VHF bands) are so foreshortened and each is formed with conductive wire for an open-type element or conductive sheet or film for a solid type dipole. The dipole has the profile of the interior cross section of either a double ridged or a single ridged waveguide and is oriented with the plane of the dipole either parallel to or transversely of the plane containing the axis of the array. A coplanar array of log periodic dipole arrays consisting of such foreshortened dipoles exhibits substantially improved gain and radiation pattern characteristics as compared to a coplanar array of conventional log periodic dipole arrays.

United States Patent [191 Kuo 1 May 8, 1973 s41 LOG PERIODIC ANTENNAWITH 3,193,831 7/1965 Yang ..343/792.5

FORESHORTENED DIPOLES Primary Examiner-Eli Lieberman [75] Inventor.gilllliltllel Chung-Shu Kuo, Cupertmo, Atmmey Norman J. UMalley et aL[73] Assignee: GTE Sylvania Incorporated, Moun- [57] ABSTRACT [63]Continuation-impart of Ser. No. 50,782, June l9,

1970, abandoned.

[52] U.S. Cl ..343/792.5, 343/802 [5l] Int. Cl. ..H0lq 11/10 [58] Fieldof Search ..343/792.5, 802

[56] References Cited UNITED STATES PATENTS 3,543,277 ll/l970 Pullara..343/792.5 3,371,348 2/1968 Simons ..343/792.5 3,573,839 4/1971 Parker..343/792.5

l -24 24 I2 W tain View, Calif.

Filed: Nov. 22, 1971 Appl. No.: 200,681

Related U.S. Application Data A log periodic antenna comprisingforeshortened dipoles, each having a profile corresponding to theinterior cross section of a hollow ridged waveguide. One or more of theelements of the array, usually those which are resonant at relativelylow frequencies (HF and low VHF bands) are so foreshortened and each isformed with conductive wire for an open-type element or conductive sheetor film for a solid type dipole. The dipole has the profile of theinterior cross section of either a double ridged or a single ridgedwaveguide and is oriented with the plane of the dipole either parallelto or transversely of the plane containing the axis of the array. Acoplanar array of log periodic dipole arrays consisting of suchforeshortened dipoles exhibits substantially improved gain and radiationpattern characteristics as compared to a coplanar array of conventionallog periodic dipole arrays.

7 Claims, 13 Drawing Figures PATENTED 81975 3,732,572

SHEET 1 UF 3 23 23c 23 22 x f INVENTOR.

-24 SAMUEL CHUNG-SHU KUO l BY 24. 3 lE-E AW R w Q ATTORNEY LOG PERIODICANTENNA WITH FORESHORTENED DIPOLES BACKGROUND OF THE INVENTION This is acontinuation-in-part of application Ser. No. 50,782 filed June 19, 1970,now abandoned.

This invention relates to log periodic antennas, and more particularlyto a log periodic array with foreshortened dipoles.

The broadband medium gain performance of log periodic dipole arrays haslong been recognized for use in the high frequency or HF (3-30 MHz) andthe very high frequency or VI-[F (30-300 MI-Iz) bands. The linearrelationship of the standard dipole to operating wavelengths, however,results in difficulty capacitive th in using such arrays in spacelimited applications. For example, at 3 and 30 MHz the dipole span is 50and meters, respectively. The need for more compact antennas has led toconsideration of foreshortening or size-reducing dipoles by varioustechniques such as series inductance loading, transmission line loadingand various types of capacitive loading of the elements as described,for example, in the article entitled Reduced-Size Log Periodic Antennasby D. F. DiFonzo, Proccedings of the 9th National CommunicationSymposium October 1963, pages 121 to 130, inclusive (IEEE). Thesetechniques, however, have not provided the required transforming action,the wideband active region and/or the high attenuation of the activeregion required for antenna gain, pattern uniformity and the front toback ratio necessary for high performance broadband operation.

An object of this invention is the provision of a log periodic dipolearray antenna having dipoles foreshortened by 35 percent or more andelectrical performance substantially equivalent to that of acorresponding conventional log periodic dipole array.

Another object is the provision of a simple technique for designing alog periodic antenna with foreshortened dipoles when the available spaceis limited.

Still another object of the invention is the provision of a dipole whichis substantially the electrical equivalent of the conventional lineardipole but which is less than two-thirds the length of the latter.

A further object of the invention is the provision of a coplanar arrayof log periodic antennas with foreshortened dipoles in which the patternbreak up and gain drop phenomenon has been eliminated or substantiallyreduced.

SUMMARY OF THE INVENTION A dipole having an outline in the shape of theinterior of a hollow ridged waveguide resonates at a substantially lowefrequency than a linear or conventional dipole of the same physicallength. Substitution of such ridged-waveguide configured dipoles forconventional linear dipoles in a log periodic array effects reduction inthe width of the array without materially changing its electricalperformance. A coplanar array of log periodic dipole arrays with therequired halfwavelength spacing between array axes has greaterinterelement spacing when constructed with such foreshortened dipoleswhich is believed to acount for the improved gain over the band ofinterest.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic plan view of aconventional log periodic dipole array;

FIG. 2 is an enlarged side view of the array of FIG. 1 taken on line 2-2of FIG; 1 and showing the feed line connections to the dipole elements;

FIG. 3 is a schematic plan view of a log periodic dipole arrayelectrically equivalent to the array of FIG. 1 and embodying theinvention, the antenna comprising a mixed array of conventional dipolesand dipoles foreshortened in accordance with the invention;

FIG. 4 is a transverse section taken on line 4-4 of FIG. 3;

FIG. 5 is a view similar to FIG. 4 showing a modified form 'of theinvention in which the plane of the dipole element extends transverselyof the axis of the array;

FIG. 6 is a plan view of a wire or open-type dipole having the outlineof a double ridged waveguide on which certain dimensional parameters areindicated;

FIG. 7 is a plan view similar to FIG. 6 in which the dipole isconstructed with solid or sheet-type conduc- I01;

FIG. 8 is a plan view of a solid dipole having the outline of asingle-ridged waveguide interior;

FIGS. 9, 10 and 11 are curves representing percent reduction ofconventional dipole length achievable with the indicated ranges ofdesign parameters of a dipole embodying the invention;

FIG. 12 is a plan vie of a coplanar dual array system in which the logperiodic antennas are formed with dipoles foreshortened in accordancewith this invention; and

FIG. 13 is an enlarged section of one of the arrays of FIG. 12 taken online 13-13 of FIG. 12.

DESCRIPTION OF PREFERRED EMBODIMENTS This invention concernsforeshortening a conventional dipole without significantly changing itselectrical characteristics by making the profile of the dipole the sameas the interior cross-sectional profile of hol' low rectangular ridgedwaveguide as defined in The International Dictionary of Physics andElectronics (Von Nostrand, 1956) at page 981. The principle upon whichthis invention is based rests on the analogy between rectangularwaveguide and the slot antenna, the latter being an efi'ective microwaveantenna whose resonant wavelength is equal to twice the length of theslot if the width of the slot is small compared to its length. Thecutofi' wavelength of the fundamental mode of a rectangular waveguide istwice the width of the guide; equating that cutofi frequency with theresonant frequency of the slot antenna, it is noted that there is acorrespondence between the waveguide cutoff and slot resonantfrequencies. Ridged waveguide has a eutofif frequency lower than that ofrectangular waveguide of the same width and height and without ridges.By analogy a lower resonant frequency for a slot antenna is achievableby shaping it with the interior profile of a ridged waveguide.Experimental measurements verify this conclusion. The efiect of loweringthe resonant frequency in this manner is to increase the effectiveelectrical length of the slot antenna without changing its physicallength. Thus the physical size of a slot antenna may be foreshortenedwithout changing its electrical length. By Babinets principle, slot anddipole antennas are analogs, and therefore the physical length of adipole is likewise capable of being foreshortened when it is formed withthe shape of a ridged waveguide. Test results indicate that, inaccordance with this invention, the physical size of the conventionaldipole can be foreshortened as much as 45% without significant change init electrical characteristics by configuring the dipole with theinterior cross-sectional profile of a ridged rectangular waveguide.

The empirical relationship for the resonant length l of a rectangularslot having a width w and centered in the transverse plane of arectangular waveguide was originally suggested by J. C. Slater inMicrowave Transmission, McGraw-Hill Book Company (1942), pages l85l87,and is as follows (accurate to within a few percent):

l= )t,/2 1 (2aw/b)t,,,,) (1) where A, is the resonant wavelength of therectangular slot, )t is the guide wavelength of )t a is the height ofthe rectangular waveguide and b is its width.

When the ratio of w/l is small, equation (1) reduces to l a M (2)Accordingly the resonant frequency of a narrow slot is approximatelyequal to the cutoff frequency of a rectangular waveguide having the samecross-sectional shape of the slot. Since the cutoff frequency of aridged waveguide is lower than that of a rectangular waveguide of thesame width and height, it follows that the resonant frequency of asimilar ridged slot is correspondingly lowered. The same is true of theanalog of the slot antenna, the dipole.

Referring now to the drawings, a conventional log periodic dipole arrayhaving an axis Z is shown in FIGS. 1 and 2 and comprises a plurality ofaxially spaced dipole elements 11 connected to a feed line J12 whichextends the length of the array along its axis. The spacing betweenadjacent dipoles and the dipole lengths increase in one axial direction,from right to left as viewed in FIG. I, in progressive increments of apredetermined constant ratio 1 over the frequency band of interest.Energy is fed to the dipoles from a feed point F and in the embodimentshown, feed line 12 comprises a coaxial line connectable to externalcircuits at connector 13 and having an outer conductor 14 and an innerconductor 15. A conductive rod 16 parallel to and coextensive withcoaxial line 12 is connected to the inner conductor 15 at feed point F.In order to insure the 180 degree phase reversal of energy fed tosuccessive dipoles as required for radiation along the axis Z, axiallysuccessive dipole elements on the same side of axis Z are alternatelyconnected to outer conductor l4 and rod 16 as shown in FIG. 2. Thedipoles are fed by the two-conductor feed line which supports a slowwave from feed point P to the opposite end of the array and energizesthe dipole or dipoles which are resonant at or near the operatingfrequency. The construction and operation of conventional log periodicantennas are well known in the art and therefore the foregoingdescription is sufficient to lay a foundation for an understanding ofthe invention described below.

The utility of a conventional log periodic antenna is often dependentupon space available for the longer dipoles in the array. As shown inFIG. I, the requirements for a space-limited application having amaximum width W available for the array cannot be satisfied with aconventional array because the three low frequency dipoles are longerthan W. This problem is solved in accordance with this invention bysubstitution in array 17 of foreshortened dipoles l8, l9 and 20 havingoutlines corresponding to the shape of the interior cross-sectionalprofile of a double-ridged waveguide, see FIG. 3, for those dipoles ofthe conventional array whose lengths exceed the limit W; the lengths ofdipoles l8, l9 and 2d are selected as described in detail below so as tobe equal to or less than the limiting dimension W. The remainder of thedipoles 11 at the high frequency end of array 17 are identical to thosein the conventional array 10 and therefore the size of the array ismodified only to the extent required by the space limits of theparticular application. Antenna 17 is a mixed array of linear andforeshortened dipoles.

Each of the elements of the foreshortened dipoles, such as dipole 18,consists of a wire-like stem 22 directly connected at one end to one ofthe feed lines and a body 23 connected to the other end of the stem.Body 23 consists of a wire-like conductor 23a con.- figured to definethe outline of a totally enclosed plane geometric pattern which, in theembodiment illustrated, is a rectangle. The dimensions of the stern andbody are selected as described below so that the foreshortened dipolebecomes essentially the electrical equivalent of the correspondinglinear dipole. The plane of each foreshortened dipole element, i.e., theplane of body 23, may be parallel to the axis Z of the array as shown inFIGS. 3 and d or, alternatively, may be oriented with the planes of theelements extending .transversely of the array axis, for example,perpendicular to the array as shown in FIG. 5. Antennas constructed withforeshortened dipoles 18, 19 and 20 in planes that are either parallelto or transversely of the array axis have been found by actual test tohave substantially the same electrical performance. In short, theelectrical performance of the foreshortened dipole is independent of theplanar orientation of body 23. The foreshortened dipoles shown in FIGS.3, 4i and 5 are formed with a wire or a wire-like conductor whichdefines the outline of each body and provides an opening 24 in thecentral part of the body.

The design parameters of dipole w, for example, are shown in FIG. 6. Theratio of body dimension B to the overall dipole length A is directlyproportional to the length-reduction factor, i.e., that factor increasesas the value of BIA increases. The limit of the ratio of IBM is dictatedby (l) the dipole resistance, which decreases as B/A' increases, and (2the spacing between adjacent dipoles in a log periodic dipole array.Other dimensions affecting the dipole length-reduction factor are thedimension D of stem 22 and the inter-body spacing S of the dipoleelements, the length-reduction factor increasing as the ratio of D/Bincreases. The optimum value of S/A for the maximum reduction of dipolelengths is approximately 0.5, the length-reduction factor generallydecreasing as the ratio S/A becomes substantially greater or less than0.5.

The effect of design parameters S, A, B and D on the dipoleforeshortening sought to be achieved may be correlated in such a manneras to enable the antenna designer readily to select dipole parameters tomeet a design requirement. Examples of such correlation of these dataare shown in FIGS. 9, l0 and 11 wherein curves plotted from such datarepresent variations in the percentage of size reduction in accordancewith variations in design parameter ratios. These curves are plottedfrom data derived from a series of foreshortened dipoles with dimensionschanged by trail and error. The antenna designer, having calculated thedimensions of a conventional log periodic array and faced with a spacelimiting requirement, determines the amount of reduction required in thelength of each oversize standard dipole in order to meet therequirement. Using the curves of the type illustrated in FIGS. 9, and11, he selects the design parameters S, A, B and D which effect thedesired foreshortening and proceeds to construct the particular dipolesin accordance with the derived data.

By way of example, assume that a linear dipole having a length of 10inches exceeds the space limits of the particular application by 3inches. The antenna designer therefore needs a 30 percent reduction ofdipole length. Referring to the curves of FIGS. 9, 10 and 11 he selectscurve 27 in FIG. 10 and determines therefore that the foreshorteneddipole should have the following parameter ratios:

S/A 0.5 BIA 0.20 D!!! 0.1

With these ratios, the designer may then select the actual dimensions ofthe foreshortened dipole which may, for example, be as follows:

7 inches 1.4 inches 3.5 inches 0. 14 inches A B S D It will be seen fromthe foregoing description that the overall length A of dipoles 18, 19and 20 of array 17 is a constant equal, to or less than W and that thedimension B of each dipole is different since the amount of reductionrequired for each of the dipoles is different. It should also be notedthat the overall axial length of the array 17 has not changed withrespect to the length of the conventional array 10 and thus the practiceof the invention achieves a reduction in he transverse dimension of thearray without necessitating enlargement of the antenna in anotherdirection as a tradeoff. The aperture of the array 10 defined by thearea within the trapezoid G,'I-I, K and L, see FIG. 1, is unavoidablyreduced in the size-reduce array 17 of FIG. 3 and to this extent thereis a diminution in gain in the latter array. In other respects, theelectrical performance of array 17 is substantially the same as that ofa conventional array. If the available space in the axial direction ofthe array permits, more foreshortened dipoles may be added to the lowfrequency end of the array to offset the aperture reduction and thuscompensate the loss of gain.

The construction of the foreshortened dipole with opening 24 in the bodyportion may be modified without changing the electrical properties bythe addition of a brace-like extension 28 of stem 22 across the openingas indicated in broken lines in FIG. 6. Arrays actually constructed withsuch dipoles not only exhibited increased mechanical rigidity but alsohad equivalent or even slightly improved electrical performance comparedto those with unbraced or fully open element bodies.

Another form of a dipole embodying the invention is shown at 30 in FIG.7 wherein the dipole outline or plan profile is identical to that ofdipole 18 and each element 31 is formed from a continuous conductiveplanar sheet or film. This construction is useful in applicationspermitting formation of the dipoles and the feed lines 12' by printedcircuit techniques on a dielectric base. The electrical performance ofthis dipole is substantially identical to that of the dipoles shown inFIG. 6.

The foregoing embodiments of the invention feature dipoles having theoutline of the interior cross section of a double-ridged waveguide. Ithas been found that dipoles may also be formed with the outline of theinterior cross section of a single ridged waveguide and such a dipole 32in sheet form is shown in FIG. 11. The electrical performance of dipole32 is essentially identical to that of dipole 30.

By way of example and comparison, an antenna comprising a mixed array ofthe type shown in FIG. 3 and a conventional array of FIG. 1 both withthe following parameters and performance characteristics, wereconstructed and successfully tested:

Conventional Log Size-Reduced Periodic Dipole Log Periodic Array MixedDipole Array Taper angle a 10 10 Scale factor 1 0.91 0.91 Number ofelements 13 13 Operating frequency range 0.7-3.0 Ghz 0.7-3.0 Ghz Antennaboom length 12 inches 12 inches Size of largest dipole element 7 inches4.2 inches Averaged E-plane radiation pattern beamwidth 59 62 Gain 9 db8 db VSWR less than 1.511 less than 1.5:1

These are performances of the size-reduced portion of the sizereducedmixed log periodic dipole array. In all other respects, electricalperformance of both arrays were the same.

The invention also has practical application in substantially improvingperformance of coplanar arrays of log periodic antennas by providing aunique solution to a gain dropout problem experienced with such arrays.Specifically, the radiation pattern of a coplanar system of conventionallog periodic dipole arrays deteriorates and the gain drops drasticallyat several frequencies periodically throughout the operating band. Thisis believed to be cause by mutual coupling between adjacent elements ofthe arrays. This problem is solved with a coplanar system shown in FIGS.12 and 13 comprising identical arrays 34 and 35 with axes Z and 2 eacharray comprising a trapezoidally shaped base layer 36 of dielectricmaterial such as fiberglass, an axially extending coaxial feed line 37,and a plurality of axially spaced dipole elements 38 formed asconductive film on layer 36 and on opposite sides of array axis.Elements 38 comprising each dipole are located on opposite sides of thedielectric sheet and are alternately connected to the feed linescomprising the outer conductor 37a and the extension 37b of the innerconductor of a coaxial line, which feed lines are on opposite sides ofthe sheet. Both arrays 34 and 35 lie in the same plane and diverge fromeach other at an angle selected to provide half wavelength spacingbetween the axes Z and Z of the arrays at the particular frequency ofoperation. All of the dipoles in each array are foreshortened asdescribed above. While coplanar arrays of the type described have beenbuilt and successfully tested without any periodic gain dropout orpattern discontinuities, such arrays formed with wiretype dipoles of thetype shown in FIG. 6 may also be used with the same advantage.

I claim:

1. A log periodic dipole antenna having an axis and comprising a pair ofaxially extending feed lines,

a plurality of axially spaced dipoles connected to said feed lines andextending in directions transversely of said axis,

at least three of said dipoles having lengths substantially less than M2where )t is the wavelength at the resonant frequency of the respectivedipole,

each of said three dipoles having two identical elements on oppositesides of said axis, each of said elements having a stem connected to oneof said feed lines and a rectangular body longitudinally connected tothe end of said stem opposite from said one of said feed lines.

2. The antenna according to claim 1 comprising a mixed array of saidlast named dipoles at the low frequency end and a plurality of lineardipoles at the high frequency end of the antenna.

3. The antenna according to claim 1 in which the plane of said body isparallel to the plane containing said axis of the antenna,

4. The antenna according to claim 1 in which the plane of said bodyextends transversely of the plane containing said axis of the antenna.

5. The antenna according to claim 1 in which the dimension of the bodyof said one element transverse to said axis is approximately equal tothe length of said stem between connections to the feed line and thebody.

6. A dual array antenna comprising first and second arrays having axesdiverging at a predetermined angle and defining the plane of theantenna,

each of said arrays comprising a pair of feed lines extending along theaxis of the array, a plurality of axially spaced dipoles connected tosaid feed lines and extending transversely of said axis, the lengths andaxial spacings of said dipoles increasing in one direction along saidaxis in progressive increments of a predetermined ratio definitive of alog periodic structure,

at least three of said dipoles having lengths substantially less than M2where A is the wavelength at the resonant frequency of the respectivedipole, each of said three dipoles having two identical elements onopposite sides of said axis, each of said elements having a stemconnected to one of said feed lines and a rectangular bodylongitudinally connected to the end of said stern opposite from said oneof said feed lines, the dipoles of said first and second arrays lying inplanes parallel to said plane of the antenna with the dipoles of onearray spaced from adjacent dipoles of the other array by an increasingdistance in the direction of divergence of said axes.

7. A multielement dipole array having an axis and comprising a pair ofaxially extending feed lines,

a plurality of axially spaced dipoles connected to said feed lines andextending in directions transversely of said axis, at least three ofsaid dipoles having lengths substantially less than M2 where A is thewavelength at the resonant frequency of the respective dipole,

each of said three dipoles having two identical elements on oppositesides of said axis, each of said elements having a stem connected to oneof said feed lines and a rectangular body longitudinally and colinearlyconnected to the end of said stem opposite from said one of said feedlines.

1. A log periodic dipole antenna having an axis and comprising a pair ofaxially extending feed lines, a plurality of axially spaced dipolesconnected to said feed lines and extending in directions transversely ofsaid axis, at least three of said dipoles having lengths substantiallyless than lambda /2 where lambda is the wavelength at the resonantfrequency of the respective dipole, each of said three dipoles havingtwo identical elements on opposite sides of said axis, each of saidelements having a stem connected to one of said feed lines and arectangular body longitudinally connected to the end of said stemopposite from said one of said feed lines.
 2. The antenna according toclaim 1 comprising a mixed array of said last named dipoles at the lowfrequency end and a plurality of linear dipoles at the high frequencyend of the antenna.
 3. The antenna according to claim 1 in which theplane of said body is parallel to the plane containing said axis of theantenna.
 4. The antenna according to claim 1 in which the plane of saidbody extends transversely of the plane containing said axis of theantenna.
 5. The antenna according to claim 1 in which the dimension ofthe body of said one element transverse to said axis is approximatelyequal to the length of said stem between connections to the feed lineand the body.
 6. A dual array antenna comprising first and second arrayshaving axes diverging at a predetermined angle and defining the plane ofthe antenna, each of said arrays comprising a pair of feed linesextending along the axis of the array, a plurality of axially spaceddipoles connected to said feed lines and extending transversely of saidaxis, the lengths and axial spacings of said dipoles increasing in onedirection along said axis in progressive increments of a predeterminedratio definitive of a log periodic structure, at least three of saiddipoles having lengths substantially less Than lambda /2 where lambda isthe wavelength at the resonant frequency of the respective dipole, eachof said three dipoles having two identical elements on opposite sides ofsaid axis, each of said elements having a stem connected to one of saidfeed lines and a rectangular body longitudinally connected to the end ofsaid stem opposite from said one of said feed lines, the dipoles of saidfirst and second arrays lying in planes parallel to said plane of theantenna with the dipoles of one array spaced from adjacent dipoles ofthe other array by an increasing distance in the direction of divergenceof said axes.
 7. A multielement dipole array having an axis andcomprising a pair of axially extending feed lines, a plurality ofaxially spaced dipoles connected to said feed lines and extending indirections transversely of said axis, at least three of said dipoleshaving lengths substantially less than lambda /2 where lambda is thewavelength at the resonant frequency of the respective dipole, each ofsaid three dipoles having two identical elements on opposite sides ofsaid axis, each of said elements having a stem connected to one of saidfeed lines and a rectangular body longitudinally and colinearlyconnected to the end of said stem opposite from said one of said feedlines.